For heating uniformity, aluminum heat-treating furnaces rely on convection heating provided by air moved in large volumes and high velocities through the heat-treating chamber containing the aluminum parts under treatment. The air must be heated to temperatures adequately high to provide the required metallurgical results.
Such a furnace has at least one baffeled plenum through which at least one blower forces air at velocities of up to 100 mph, and the plenum contains at least one electric resistance heater.
To provide the necessary heat such a heater typically may have a length in the area of three feet and must support a resistance coil element consuming power in the area of 15,000 to 30,000 watts. For good efficiency the coil should expose as much of its surface to the air as is consistant with structural stability when stressed by the force of the air blast.
The maximum amount of heat exchange surface is provided when an electric resistance heating wire is coiled in the form of a porcupine coil such as was apparently first proposed by Loguin in his U.S. Pat. No. 1,171,059.
However, insofar as is known the porcupine coil configuration has never been used for a metallurgical convection heat-treating furnace where the necessary size of the coil, both as to wire gauge and length, presents problems concerning the mounting of the coil. The use of a porcupine coil has been suggested by the prior art for use in comparatively low-voltage hot-blast guns as evidenced by the Pricenski et al U.S. Pat. Nos. 3,551,643 and the Haglund patent 4,207,457.
The main object of the present invention is to provide an electric resistance heater capable of enjoying the advantages of the porcupine coil configuration and adapted for commercial use in such a metallurgical convection heat-treatment furnace, for example, where the heater must use heavy gauge resistance wire with a coil length in the the area of three feet or more and mounted so as to be stable and long-lived under the high blast velocities incidental to such service. In such an installation the heater must operate horizontally and transversely with respect to the air blast.